1. Field of the Invention
This invention is in the field of refrigeration evaporators with drain pans including means for detecting accumulated water or ice therein. The term "moisture" as used in this specification and claims means "water" or "ice" or "water and ice".
2. Description of the Prior Art
The process of cooling air frequently results in the condensation of water from the air onto the cooling surface. Where the coolant is above the freezing point of water, the water deposits on the cooling surface as liquid. The water flows by gravity from the cooling surface into a pan provided under the surface and out of the pan into an outlet fitting connected to the pan at its lowest point. The outlet fitting is usually connected to a pipe which conveys the water to sewer or drain.
Where the cooling surface has a temperature below 32.degree. F., the freezing point of water, frost accumulates on it by a process which is the reverse of sublimation, proceeding from water in the vapor state to water in the solid state; known as frost in the fluffy as-deposited condition, and as ice in its dense thawed-and-frozen condition; without traversing the liquid state. The cooling surface is most generally a finned coil, although a plate coil or bare tube coil can be used. Over a period of cooling operation enough frost can, and frequently does, accumulate to block air passages through the cooling coil. When the air passages in the coil are blocked by frost, the effectiveness of the coil for the purpose of cooling air is sharply reduced.
At or before this time, it is generally desirable to defrost the surface, restoring the air passages to their original, unfrosted condition. The source of heat for defrost may be from electric heaters or from hot gas. The means for defrosting are well-known to those skilled in the refrigeration art and are exemplified by matter shown in U.S. Pat. Nos. 2,718,764 and 3,464,226. The water resulting from the thawing of the frost on the coil runs into a pan provided for this purpose and is conveyed to a sewer via a drain fitting and drain line.
The drain line from the drain pan to the sewer frequently traverses a cold area and therefore must be heated to prevent the meltage from re-freezing. Should the drain line be blocked; by ice, through failure of the heater; by dirt, or any other way; meltage will collect in the drain pan and may freeze on termination of the defrost and resumption of the refrigeration cycle. As additional meltage from each successive defrost operation collects and freezes, a large accumulation of ice in the drain pan and around the coil may occur.
Occurrences of this sort in frozen food display cases and in cooling coils used in bakeries and food processing plants are frequent, primarily because of dirt accumulation which blocks the drain, combined with poor maintenance. Thawing out a completely iced-up evaporator in a freezer box or display case is a time-consuming and expensive job for a serviceman and inevitably results in much mess and lost sales in the store using the display case, or lost production time where the evaporator is used for cooling or freezing products. It is apparent that the use of floats or mechanical devices to detect water or ice where the temperature is below 32.degree. F., the freezing point of water, will not be effective.